1. Field of the Invention
The invention relates to a power output apparatus, a hybrid vehicle provided with that power output apparatus, and a control method of a power output apparatus.
2. Description of the Related Art
Japanese Patent Application Publication No. 2004-340102 (JP-A-2004-340102), for example, describes a power output apparatus that includes an internal combustion engine, a planetary gear set that has a planetary carrier connected to a crankshaft of the internal combustion engine, a first electric motor that generates electric power and is connected to a sun gear of the planetary gear set, a second electric motor that outputs power to a ring gear shaft that serves as a drive shaft and is connected to the ring gear of the planetary gear set, and a battery that supplies and receives power to and from the first electric motor and the second electric motor. With this power output apparatus, a supply of fuel to the internal combustion engine is prohibited from being stopped (i.e., a fuel cut is prohibited) to suppress degradation of a catalyst that purifies the exhaust gas of the internal combustion engine when the temperature of the catalyst is equal to or above a set temperature. If there is a demand to decelerate the drive shaft while a fuel cut is being prohibited, the internal combustion engine is controlled so that the output torque becomes a value of 0, and the rotation speed (also simply referred to as “speed” in this specification) of the internal combustion engine is maintained at the value at that time by the first electric motor, while the second electric motor is controlled to output regenerative braking force. Also, Japanese Patent Application Publication No. 2007-084034 (JP-A-2007-084034), for example, describes this type of power output apparatus. When the brake pedal is depressed while a catalyst degradation suppression flag is set to a value of 1 such that a request is being made to suppress catalyst degradation, the described power output apparatus first controls a motor (i.e., the first electric motor) to decrease the speed of the internal combustion engine to a predetermined target speed (such as 800 to 1000 rpm) while the internal combustion engine continues to fire, and then the internal combustion engine is operated on its own.
Like the technology described in JP-A-2004-340102, having the first electric motor maintain or increase the speed of the internal combustion engine when there is a demand to decelerate the drive shaft while a fuel cut is being prohibited in order to suppress catalyst degradation enables braking force from the so-called engine brake to be output to the drive shaft. It is also possible to inhibit the battery from being overcharged by the electric power regenerated by the second electric motor because the first electric motor consumes electric power. However, if the speed of the internal combustion engine is maintained even though there is a demand for deceleration, it may feel odd to an occupant of the vehicle provided with the power output apparatus. Therefore, if there is a demand to decelerate the drive shaft while a fuel cut is being prohibited to suppress catalyst degradation, the first electric motor is preferably controlled to reduce the speed of the internal combustion engine, like the technology described in JP-A-2007-084034. However, when decreasing the speed of the internal combustion engine, the first electric motor functions as a generator and generates electric power. Therefore, when the amount of electric power that is allowed to be charged to the battery is being limited due to the state of the battery, it becomes necessary to prohibit the speed of the internal combustion engine from being reduced using the first electric motor in order to suppress battery degradation due to overcharging. And if the speed of the internal combustion engine is no longer able to be reduced using the first electric motor in this way, the speed of the internal combustion engine will not quickly decrease even though there is a demand for deceleration, which may feel odd to the occupant of the vehicle provided with the power output apparatus.